Democratic presidential candidate Bernie Sanders is set to pick up one of his biggest endorsements yet Thursday from the powerful Communications Workers of America union, sources told NBC News' Andrea Mitchell.

The group represents about 700,000 workers nationally, making it by far the largest union to back Sanders yet. CWA's endorsement, which will be announced at a news conference at 11 a.m. Thursday at the union's headquarters in Washington, comes as Sanders has lost out on a string of major union endorsements to Democratic front-runner Hillary Clinton, whose campaign now claims the support of unions representing 12 million workers.

Larry Cohen, CWA's former president, joined Sanders' campaign as a top labor adviser shortly after stepping down in June. The union has been hinting at a possible Sanders endorsement for months, saying the decision would come only after members voted in an online poll. The national union did not issue an endorsement in the 2008 Democratic primary between Clinton and Barack Obama.

With only two members of Congress in his corner, this is one of Sanders' most important endorsements yet. CWA boasts it has more than 300,000 active and retired members in the states that hold primaries and caucuses between now and April 1, who could be mobilized to support Sanders.